On the other hand, some troubles may occur with the other chemicals, such as:

The solvent-based (non-aqueous wet) developers in PT;

The contrast aid paints and the water-based magnetic ink concentrates in MT.

These are called "two-phase" type chemicals, as they comprise a liquid phase and a solid phase.

These products will give satisfactory results only if there is a HOMOGENEOUS suspension of THE ENTIRE solid phase in the liquid phase, so as to get a uniform dispersion before use.

This paper is useful to users as well as to the manufacturers/suppliers.

2 | THE POTENTIAL RISKS

Should “two-phase” type PT or MT materials have not been homogenized prior to use, for instance, by a thorough shaking, the inspection reliability may be questionable. Here are the potential risks.

2.1. Developer

If not homogeneous when sprayed, the sprayed developer will contain too few or too much powders and additives.

If too few, more developer will be sprayed to get the right layer, which will, then be too wet and will need too much time for drying; then the developer will run leading to fuzzy indications or even to discontinuities not detected, especially the small ones.

If too much, the developer will be more viscous, hence more difficult to spray. The layer may be too thick and uneven: some discontinuities may go undetected, especially the smallest ones.

2.2. Contrast aid paint for MT

A trouble very similar to that met with developers may occur.

If the paint when sprayed is too low in pigments, resins, additives, more paint will be used to get the right layer. This layer will be too wet and will need too much time for drying. The detection medium will be applied later than anticipated, hence making the entire inspection process more time-consuming. Furthermore, an orange-peel effect may occur, detrimental to the detection performance.

If too high in pigments, resins, additives, the paint is more viscous and more difficult to spray. The layer may be too thick (more than 50 µm) and uneven: some discontinuities may go undetected, especially the smallest ones. Again the orange-peel effect is likely, with the same detrimental result on sensitivity.

2.3. Water-based magnetic ink concentrate

A non-homogeneous water-based magnetic ink concentrate contains not enough, or more than necessary, magnetic particles.

If too few, many discontinuities may go undetected; maybe no indication would be seen!

If too much, the background will be dramatically increased, making it far more difficult to see faint indications. Once again, sensitivity would be impaired.

3 | CAKING

Unfortunately, a thorough and long shaking of cans is not always enough to get a homogeneous “two-phase” type material. Once again, the ENTIRE content shall be in suspension. However, when cans are stored for too long a time, the settled solid phase becomes a cake in the bottom of the can.

We all met such a trouble, and we had to use a wooden or plastic stick to try to get out of the mess!

We even saw people using hand-held drills fitted with a propellor. This is very dangerous due to the low flash point of the solvents used in these products and the build-up of explosive vapour, as the drills’ motor is not explosion-proof!!

To prevent this caking, some products' designers add an anti-caking agent. This may help in preventing the problem...for a while. However, quite often, one year storage without any can shaking is enough for caking. That is why some suppliers specify a 12- or 18- month shelf life for their products.

What can be done?

4 | CHOICE OF PACKAGING MATERIALS

Some manufacturers use tinplate containers. Is this the right idea?

Certainly not, for the underneath point.

Along the years, we have noticed that caking at the bottom of cans occurred more likely in metal cans than in plastic ones.

We think this comes from the Van der Walls’ attractive forces between moleculeswhich ease gluing of particles together, as we already wrote on our Website about the dry developers(*).

This caking is almost never seen in oil (aliphatic hydrocarbons)-based magnetic inks: a proof that the liquid phase plays a part in the cake’s build-up?

In fact, the liquid phase of solvent-based (non-aqueous wet) developers is usually based on 2-propanol and sometimes acetone; the liquid phase of contrast aid paints is often acetone-based and the liquid phase of water-based magnetic ink concentrates...is water.

2-propanol, acetone and water all are polar solvents while aliphatic hydrocarbons are not.

Tinplate, as any metallic alloy, conducts electricity; the packaging of choice for these products is thus plastic, which does not conduct electricity.

Transparent plastics allow users to be sure that ALL of the solid phase is well put back in suspension.

Made of flexible plastic, the ¼ litre to 2 litres (1/2 to 4 Imperial pints) doses of water-based magnetic ink concentrates make it easy to break the cake just by squeezing them.

When making a homogeneous suspension is not that easy, a wide-neck can makes it easier to put back the settled solid particles in suspension.

Last point: use polyethylene or polypropylene cans and NOT polyvinyl chloride or any halogen-based (chlorinated or fluoro-chlorinated) plastic to prevent any pollution by halogens on long-term storage. Suppliers generally follow this rule, for at least one good reason: chlorinated or fluoro-chlorinated cans are more expensive!

Prefer quite thick plastic cans. This is the supplier’s choice, users have no say in that. A thick wall lowers solvents “perspiration”, i.e. the solvent going out of the can through the plastic. This is of the utmost importance for acetone-based products, but also for hydrocarbon-based MT products. Another example where “cost-cutting mind” may have very important technical drawbacks.

To prevent such a “perspiration” of chemicals, some plastic cans comprise three layers of plastic, the middle one being very thin, made of a fluorinated polymer. A fluorinated chemical in contact with low-in-halogens chemicals may be a concern. However, these cans being far more expensive than “simple” polyethylene or polypropylene cans....the problem simply does not exist!!!

5 | CONCLUSION

While metallic cans are convenient for most of the PT and MT chemicals, for solvent-based (non-aqueous wet) developers, contrast aid paints and water-based magnetic ink concentrates, we recommend wide-neck, transparentpolyethylene or polypropylene cans.